1. Field
Disclosed herein is an optical probe containing integrated oxygen, temperature, and pressure sensors. The probe is particularly suitable for determining the concentration of oxygen, particularly the partial pressure of oxygen, in an enclosed space, such as a fuel tank, cargo hold, passenger compartment, or other space in a vehicle, such as an aircraft, ship, boat, land vehicle, or other military, commercial, or aerospace vessel.
2. Description of Related Art
Since 1959, a number of aircraft fuel tanks have unexpectedly exploded. Typically, the explosions occurred when an unknown ignition source ignited the fuel/vapor mixture in the fuel tank. Fuel/vapor mixtures are created during consumption of fuel within the fuel tank by engines of the aircraft. The consumed fuel leaves a space within the tank which generally fills with atmospheric air containing oxygen. The presence of both a flammable gas and the fuel/vapor mixture within the space creates the potential for an explosion within the fuel tank upon ignition. The industry has responded with various methods and apparatuses, as discussed in Air Safety Week, Vol. 15 No. 16, Apr. 16, 2001, “Fatal Explosion Highlights Hazard of Flammable Vapors in Fuel Tanks.” In addition, recent combat experience with fixed wing and rotary aircraft suggests that fuel tanks are susceptible to penetration by hostile fire, and in particular by shrapnel and small arms fire, which creates both an ingress for oxygen and a potential ignition source. Moreover, if lightning penetrates the wing or fuel tank of an aircraft as the result of a lightning strike, it can be a significant source of ignition, particularly in aircraft constructed using composite materials.
One method which reduces fuel/vapor combustion includes the elimination of combustible gases from the fuel tank. This method fills space within a fuel tank with an inert gas. The presence of the inert gas within the fuel tank deprives the fuel/vapor mixture of a flammable gas necessary for combustion. Nonetheless, the need to continuously fill the fuel tank with an inert gas and the attendant high costs associated therewith do not make this an attractive alternative for aircraft manufacturers.
A more efficient method in accordance with the prior art includes flooding the tank with inert gas when oxygen levels become high. This method requires continually measuring oxygen levels in a fuel tank. However, in order to accurately determine the oxygen concentration, either the temperature level of the oxygen sensor must be kept constant, or the temperature of the sensor must be measured in real time and taken into account in calculating the oxygen partial pressure from the sensor signal. Similar concerns arise with regard to the pressure in the vicinity of the oxygen sensor. However, temperatures and pressures within an enclosed space, such as fuel tanks in vehicles, can fluctuate over time depending on the outside temperature. In addition, spatial fluctuations in temperature and pressure within the fuel tank can occur. Moreover, these fluctuations can impact the ability of the monitoring and control electronics to accurately determine the oxygen level from the data obtained from the oxygen sensor.
Prior art attempts to address similar issues involved keeping the temperature of a gas sensor constant include heating the gas sensor with electric resistance heaters when the temperature is low. However, these methods are not suitable for use in fuel tanks, as electrical current applied to the electrical resistance heaters may potentially ignite the fuel/vapor mixture within the tank, again making this an unattractive option for aircraft manufacturers. Other attempts to provide fuel tank inerting systems and sensors include those described in U.S. Pat. Nos. 6,634,598; 6,904,930; 6,925,852; and 7,231,809. An oxygen probe containing a temperature sensor has also been developed, but the absence of an integrated pressure sensor requires that the system use pressure information from the aircraft pitot tubes. Such a system provides pressure information that is representative of the pressure in the fuel tank, but is based on the pressure outside of the aircraft. Alternatively, a pressure sensor system built into the fuel tank at a fixed position can provide erroneous information when the aircraft is not level. As a result, the pressure measurement from such systems is not accurate information about the ullage pressure in the vicinity of the probe, where the oxygen level is actually being measured.
Therefore, a need exists for a method and apparatus which provides accurate information about the temperature and pressure environment in the vicinity of the oxygen sensor, and that does not require potential sources of ignition, particularly in a vehicle fuel tank.
In addition, some vehicles, such as commercial and military aircraft, contain areas, such as cargo holds, passenger compartments, and the like, that may, expectedly or unexpectedly, contain materials capable of supporting combustion. Such areas can be equipped with fire suppression systems, which often are manually operated from the flight deck when an indication of a fire is received, generally from an increase in temperature in the cargo hold. However, there is typically no access to the cargo hold from the flight deck, and even if the fire suppression system is effective, temperature in the cargo hold may remain elevated for a considerable period of time. As a result, with such a system there is limited ability of the flight crew to determine in the short term whether the fire suppression system has been successfully deployed and has been effective in controlling the fire. Accordingly, there remains a need for a monitoring system that can allow a more rapid and accurate determination of whether fire suppression and control systems have been effective.
Fire suppression in passenger compartments provides a particular challenge, requiring precise control of the type and amount of fire suppression gas introduced, so as to decrease oxygen available for combustion while maintaining sufficient oxygen for life support of the passengers. Accurate monitoring of oxygen concentration in such a space is essential.
One technique that might be suggested as suitable for fuel tank inerting or monitoring is the On Board Inert Gas Generating System (OBIGGS system). This system processes pressurized air through hollow fiber membranes to obtain a nitrogen enriched air, which can be used as an inerting gas. However, the implementation of this system is not optimal, because of the lack of an appropriate sensing/control system. As a result, attempts have been made to operate OBIGGS equipped aircraft with the system constantly operational (i.e., continuously supplying nitrogen to the ullage of the aircraft fuel tanks). Such an operation, however, incurs a significant fuel penalty. Accordingly, there remains a need in the art for a sensing/control system that allows an inerting system such as OBIGGS to be operated when necessary (i.e., when the oxygen partial pressure in the ullage of the fuel tank reaches a predetermined value) and to be idled when operation is not necessary, thereby increasing fuel economy. Such an idled mode includes heating of the system to prevent freezing of moisture in the system.
Accordingly, there remains a need in the art for a probe that provides accurate, localized information about oxygen concentration, pressure, and temperature, and that is capable of operating under the stringent environmental conditions found in, e.g., an aviation fuel tank. These conditions include operation under widely varying temperatures, operation under low temperatures, operation while exposed to the components of fuels, and particularly while exposed to the hydrocarbons in various aviation fuels, such as jet fuels, and operation under vibration.